1. Field of Invention
The invention relates to a blown film extrusion system that generally includes an annular die gap, a calibration basket having film-guide elements, and a flatness unit or nip rolls.
2. Description of the Prior Art
Such blown film extrusion lines are known from the prior art and are already in use since long. Such lines are supplied with plastic in granulated form, which is then plasticized in extruders under the effect of high pressure to form a viscous mass. This mass, which is of high temperature due to the pressure, is provided with an annular shape in a blown film die and is discharged from the blown film die through a tubular die. The mass forms a film tube immediately after leaving the tubular die. However, the diameter of this film tube can be changed since the latter has not yet cooled down completely. Usually, the diameter is increased by blowing compressed air into the interior of the film tube. In order for the film tube to have a constant diameter at all times, it is guided at a distance from or directly along film-guide elements. This arrangement of the film-guide elements is referred to as “calibration basket” in the field of blown film extrusion lines. After passing through the calibration basket, the film tube that is now solidified is guided along additional film-guide elements, which flatten the tube. This flatness unit supplies the film tube to a squeezing device so that the film tube forms a two-ply film web. The term “squeezing” is also meant to connote both the complete and incomplete flattening of the film tube. Processing steps such as cutting the film longitudinally along the folded edges can further follow the incomplete flattening process of the film tube.
The film-guide elements of the calibration basket can be two-dimensional elements made of a slippery material, but they can also be rollers. Teflon rollers, felt rollers, pearl strips, and brush-like or broom-like elements can be used as film-guide elements. The objective of all these measures is to guide the film reliably and to apply a force on the film to the fullest extent while also preventing any damages to the still hot, freshly extruded film. For quite some time, calibration baskets are therefore being used, which comprise film-guide elements, which comprise an air cushion on the side that faces the tube and which guide the tube in a contact-free manner.
For this purpose, these tube guide elements are penetrated by bores, to which compressed air is applied on that side of the film-guide elements that is turned away from the film tube. The compressed air flowing through the bores, which have a diameter of 0.5 mm by way of example, keeps the film tube at a distance so that the latter is guided in a contact-free manner. This helps prevent damages to the film tube.
However, the bores must be provided at a certain distance from each other so that the film-guide elements do not lose their stability. But this results in the air cushion, which guides the film tube, not acting uniformly on the periphery of the film tube. This adversely affects the accuracy of guidance. The result is greater inaccuracies in the diameter of the film tube. The film tube also tends to flutter due to the non-uniform application of compressed air.
It is therefore suggested in EP 1 488 910 A1 to use a porous or micro-porous material for designing the film-guide elements, which porous or micro-porous material extrudes air through a plurality of small holes and thus produces a particularly suitable air cushion. A porous, preferably a micro-porous material—for example, sintered material, is suggested to be used as the material, which contains this plurality of small holes.
Such materials comprise a plurality of continuous pores. It is therefore possible to apply compressed air to that side of the film-guide elements that is turned away from the film tube. This compressed air then forms an almost uniform air cushion on that side of the film-guide elements that faces the film tube. This air cushion can be used to guide the film tube with high quality.
Preferably sintered material is used as the material that has these properties.
The aforementioned types of film-guide elements usually guide a portion of the periphery of the tube. That surface of the film-guide elements that faces the tube and is used to guide the film or that surface of the film-guide elements that supports the air cushion guiding the tube is curved in order to mate perfectly with the round peripheral surface of the circle. This can be seen clearly, for example, in EP 1 488 910 A1 mentioned above.
If the format—thus in this case, the diameter of the cross-sectional surface—of the film tube is to be changed in such a device, it is necessary to change the relative positions of the film-guide elements. This is usually achieved by changing the position of the film-guide elements in the radial direction of the film bubble. For this purpose, the film-guide elements in EP 1 488 910 A1 are provided with individual adjusting means. These adjusting means must be operated carefully in order to ensure that the round shape of the film cross-section is retained.